>Mark
>
>Decent focal plane shutters have complex curtain movements. For
>instance, as the first curtain travels it will accelerate, so it'll be
>moving faster near the end of it's travel. This will make the effective
>exposure vary across the film plane. To compensate for this, the second
>curtain's movement is such that the "slit" widens as it travels, to
>compensate for the acceleration of the first curtain.
I am discovering just how complex these cameras are inside :)
Yes, I had assumed that the curtains were matched to have the same
accelerations, but this would be harder to measure. If we had a
high-speed video setup in our lab, I'd borrow that, but we don't.
>What does this mean? The measurements of the "speed" of the curtain and
>the width of the "slit" will vary depending upon the point at which the
>reading it taken.
Yep, the slit width and instantaneous curtain speed will vary, but if the
acceleration profiles of each curtain are identical, then the total
exposure time will be constant across the film plane.
I'm just using two phototransistors to measure the time at which each
curtain passes a point, effectively measuring the average curtain speed
across the film plane. This is how I assumed most shutter testers work,
since the value reported in the repair manuals is a single time.
>The worst case is the old focal plane shutters in Speed Graphic press
>cameras. They had single curtains that had fixed-width slits (just a
>rectangle cut out of the material) that could produce very uneven
>exposure across the huge film area. They weren't "self-capping" eihter,
>so if you cocked the shutter without replacing the dark slide, you'd
>expose the film to light again! No wonder the Rollei Twin lens cleaned
>up the press market whan it appeared...
>
>
>Vaughan
Mark Marr-Lyon
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